Interpretive Summary: The emerald ash borer (EAB) has now become a devastating forest pest in North America, killing hundreds of millions of native North American ash (Fraxinus spp.) trees since its discovery in Michigan, USA in 2002. The parasitic wasp (Tetrastichus planipennisi) was previously introduced from China to North America for EAB biological control. Since its introduction, this parasitic wasp has established in many areas in the US, but parasitism rates have been lower than anticipated. In contrast, parasitism rates by a group of native parasitic wasp (Atanycolus spp.) have increased in recent years in some areas in Michigan. In a field experiment we examined how bark thickness of ash trees can affect parasitism by the introduced Chinese parasitic wasp and the native wasp (Atanycolus spp.). We found that the introduced Chinese parasitic wasp was not able to attack EAB larvae in trees with bark thicker than 3.2 mm [or diameter at breast height (DBH) BH above 11.2 cm], while the native parasitic wasp were able to parasitize EAB larvae under the thickest bark tested, 8.8 mm (57.4 cm DBH). This suggests that the introduced Chinese parasitic wasp will only be an effective biological control agent in younger trees or stands, below 12 cm DBH. Releasing the introduced Chinese parasitic wasp near the leading edge of EAB infested forests may have too little impact on EAB populations because many ash trees are likely to be too large. We recommend that the introduced Chinese wasp be released in areas where ash is regenerating or invading new areas and where ash stands are dominated by smaller trees. This should maximize the effectiveness of this species. This limitation of the introduced Chinese parasitic wasp for biological control of EAB suggests that new species of larger parasitic wasps from the native range should be vigorously sought for release to North America.

Technical Abstract:
Factors that influence the effectiveness of an introduced biological control agent must be well understood to make management decisions in a successful biological control program. Information on agent biology is also required to determine how new exotic species will interact with native natural enemies. The larval endoparasitoid Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae) was introduced from China to North America for biological control of the emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) (EAB). Since its introduction, T. planipennisi has established in many areas, but parasitism rates have been lower than anticipated. In contrast, parasitism rates by native Atanycolus spp. (Hymenoptera: Braconidae) have increased in recent years in some areas. In a field experiment we examined how bark thickness of ash trees can affect parasitism by T. planipennisi and Atanycolus spp. We found that T. planipennisi was not able to attack EAB larvae in trees with bark thicker than 3.2 mm (or DBH above 11.2 cm), while Atanycolus spp. were able to parasitize larvae under the thickest bark tested, 8.8 mm (57.4 cm DBH). This suggests that T. planipennisi will only be an effective biological control agent in younger trees or stands, below 12 cm DBH. Releasing T. planipennisi near the leading edge of EAB invasion may have too little impact on EAB populations because many ash trees are likely to be too large. We recommend that T. planipennisi be released in areas where ash is regenerating or invading new areas and where stands are dominated by smaller trees. This should maximize the effectiveness of this species. This limitation of T. planipennisi for biological control of emerald ash borer suggests that new species of parasitoids from the native range with longer ovipositors should be vigorously sought for release to North America.